2018 Civic Si P0420 Code - Catalyst System Efficiency Below Threshold

ClearTheCode guidance (Honda · Diagnostics & codes)

Summary: For 2018 Civic Si P0420 Code - Catalyst System Efficiency Below Threshold, work through these steps on your 2016+ vehicle:

1. Confirm the symptom — note when it happens (cold start, highway, after fuel fill, etc.).
2. Scan codes — save all pending/stored DTCs; don't clear until you've recorded them.
3. Check TSBs & recalls — look up your VIN for open campaigns and known fixes.
4. Inspect the obvious — related fuses, connectors, fluid level, and recent repairs.
5. Test the common fix — follow model-specific service procedures; one change at a time.
6. Escalate if needed — burning smell, no brakes, overheating, or stalling in traffic → shop visit.

Owner reported: My 2018 Honda Civic Si just threw a Check Engine Light, and when I scanned it, I got a P0420 code. This indicates 'Catalyst System Efficiency Below Threshold (Bank 1)'.

I've noticed a slight decrease in fuel economy lately, but no major performance issues like loss of power or strange smells. The car has about 75,000 miles on it.

I'm hoping it's not the catalytic converter itself, as that's an e…

This is educational guidance, not a substitute for hands-on diagnosis.